Control of Particle Formation

ABSTRACT

A method of prilling or spray drying comprising calculating and controlling the viscosity of a shear-thinnable fluid stream at a particle-forming section of a dispersion device. The method comprises measuring the static head of fluid in the dispersion device, providing a pressurized blanket of inert gas over the fluid, mechanically agitating the fluid in the dispersion device, calculating the viscosity at the particle-forming section and controlling the viscosity by adjusting the speed of mechanical agitation.

FIELD OF THE INVENTION

This invention relates to an method of particle formation. Moreparticularly, it relates to a method of prilling or spray-drying ashear-thinnable mixture by calculating and controlling the viscosity ofthe mixture just prior to formation of particles.

BACKGROUND OF THE INVENTION

Prilling and spray drying are used widely in the production of sphericalparticles having uniform size and physical characteristics. Prilling isparticularly useful in the fertilizer industry. The advantages ofprilling are well known in the art, and include: high percentage ofdesired product size and thus little recycle, reduced moisture contentleading to reduced drying, and excellent sphericity. One frequentproblem in prilling relates to processing thick mixtures resulting froma high concentration of a non-melting component in a melt. It may becomedifficult to flow the resulting thick mixture through conventionalprilling systems.

U.S. Pat. No. 7,175,684 addresses this problem by effecting shearthinning by mechanical agitation in the prill head. The thinned mixturecan be prilled through a modified prill head in a conventional manner.The features and disclosures of the '684 patent are incorporated hereinin their entirety by reference thereto.

Other approaches have been disclosed to solve this problem of a thickmixture. For example, methods have been devised to manage thiscomplication by minimizing reaction time. GB 1,481,038 teaches a simpleconcept of severely limiting the processing time (the period between thetime the melt is fed into the mixer to the time the drops are dischargedfrom the prilling apparatus) to 10 seconds or less so that the mixturecan be prilled before any detrimental effects develop. U.S. Pat. No.3,617,235 teaches the use of larger-sized solid particles to slowreaction prior to prilling. U.S. Pat. No. 4,323,386 teaches a method ofmanaging the addition of reagent, delaying full addition of ingredientsuntil just prior to prilling, again to preclude reactions. U.S. Pat. No.3,856,269 discloses a mixing apparatus to facilitate prilling byproviding very rapid but adequate mixing of fertilizer ingredients priorto prilling in a standard perforated spinning bucket. These methods addcost and complication to the prilling operation, however, by requiringfeedstocks with narrow specifications or the expense of engineering theproduction system to achieve very brief residence times, or elsecompromising a desirable property by curtailing the extent of reaction.

Another approach to prilling thick mixtures is to provide equipment thatwill force the flow of the slurry. SE 70,119 teaches a vertical screwmachine to blend ammonium nitrate melt with ammonium sulfate solid; thepressure developed by the screw and static head is supplemented byinjection behind the spray nozzle. DE 2,355,660 teaches a prill headincorporating a stirrer-impeller mechanical device similar to acentrifugal pump whereby the slurry is introduced at the center andforced under pressure out holes on the circumference of the prillingdisk. These mechanical devices involve considerable cost inconstruction, and the abrasive nature of fertilizer slurry will wear theclose clearances necessary for efficient pumping with resultingsignificant maintenance expense.

Spray drying using a two-fluid nozzle is disclosed in U.S. Pat. No.4,705,535. The nozzle is adjustable to provide a substantially constantmixing energy for atomization of the liquid. Mixing energy is determinedfrom the mass flow of gas and of liquid to the nozzle, gas pressure atthe point of feed to the nozzle, and pressure in the vessel into whichthe spray is discharged. This reference is silent, however, with respectto modification of characteristics of the fluid to the nozzle.

U.S. Pat. No. 4,013,504 teaches a spray-drying apparatus comprising ahigh-speed rotary atomizing head having circumferentially spaceddischarge orifices and spiral vanes on the exterior portion of the head.A film of slurry from the orifices flowing over the vanes is subjectedto high shear stresses which materially improve the viscosity of theproduct and disperses particles in an efficient manner. This referenceacknowledges the problem of viscosity and use of shear, but does notdisclose control of flow through the orifices.

The present invention avoids the higher costs, maintenance andcomplications of the known art as well as the disadvantages ofapproaches based on curtailed reactions, and builds on theshear-thinning feature of U.S. Pat. No. 7,175,684.

SUMMARY OF THE INVENTION

The present invention broadly comprises a method of calculating andcontrolling the viscosity of a shear-thinnable fluid stream at aparticle-forming section of a dispersion device by providing a measuredstatic head of shear-thinnable fluid within the dispersion device,providing a pressurized blanket of inert gas over the fluid at ameasured gas pressure, mechanically agitating the fluid in thedispersion device at a measured agitation speed; calculating theviscosity of the agitated fluid, and controlling the viscosity as thefluid passes through the particle-forming section by adjusting theagitation speed.

A more specific embodiment comprises a method of calculating andcontrolling the viscosity of a shear-thinnable fluid stream at a prillhead by providing a measured static head of shear-thinnable liquidwithin the prill head, providing a pressurized blanket of inert gas overthe liquid at a measured gas pressure, mechanically agitating the liquidin the prill head at a measured agitation speed, calculating theviscosity of the agitated liquid as it passes through the particleforming section, and controlling the viscosity of the liquid inside theparticle forming section by adjusting the agitation speed.

An alternative embodiment comprises a method of calculating andcontrolling the viscosity of a shear-thinnable fluid stream in a spraynozzle by providing a measured static head of shear-thinnable fluidwithin the nozzle, providing a pressurized blanket of inert gas over thefluid at a measured gas pressure, mechanically agitating the fluid inthe nozzle at a measured agitation speed, calculating the viscosity ofthe agitated fluid, and controlling the viscosity of the fluid byadjusting the agitation speed.

A specific use of the invention comprises addition of ammonium sulfateto an ammonium nitrate melt, reaction of the consequent mixture to forma double salt ammonium sulfate nitrate, and prilling the resultantshear-thinnable melt slurry in the present process to yield prillshaving excellent strength, sphericity and storage properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a broad schematic representation of the present invention.

FIG. 2 shows a prill head design useful in the practice of theinvention.

FIG. 3 shows a spray-drying apparatus useful in the practice of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “prilling” as used herein refers to formation of solidparticles or “prills” in an open tower via solidification as dropletsfall from a prill head. A prill head is the apparatus at the top of aprill tower which divides the molten material into the streams fromwhich the prills form.

The term “spray-drying” as used herein refers to dispersal of a solutionor slurry into a stream of hot gas in a manner to vaporize a volatilesolvent and produce residual solid particulates. Spray-drying isdistinguished from prilling by the substantial presence of a volatilesolvent in the solution or slurry.

The term “shear-thinning” as used herein refers to the phenomena ofdecreasing viscosity with increasing shear rate. Not all mixturesexhibit shear-thinning and it cannot be predicted which mixtures willand which will not possess this behavior.

The term “shear-thinnable mixture” as used herein refers to a systemcomprising at least two components in which the first component is orforms a molten melt and at least one second component which incombination results in a mixture which has high viscosity anddemonstrates shear-thinning behavior. A “shear-thinnable mixture” mayinclude a melt slurry wherein the molten limited meltability and/orlimited solubility mixture contains solid particles.

The schematic representation in FIG. 1 can generally be considered toembrace a cross-section of either the prilling or the spray-dryingembodiment of the invention. A dispersion device 10 contains a fluidfrom which particles are to be formed as well as a pressure gauge, ashear-thinning apparatus and a particle-forming section. Particles areformed from section 11, which may be configured in any suitable mannereither for prill formation or for spray drying as described elsewhereherein. The fluid, having a surface represented by 12, enters theapparatus through conduit 13. The level of the fluid surface in thedevice is controlled between upper and lower limits established by levelgauge 14 between the entry conduit and the shear-thinning apparatus. Aninert gas, for example nitrogen, is provided through conduit 15 anddetermines pressure in the apparatus through pressure gauge 16.Shear-thinning of the fluid is effected by an apparatus comprisingagitation device 17 which is rotated within the device by motive deviceand shaft 18. The agitation and motive devices may be of any suitableconfiguration described herein or generally known in the art; it iswithin the scope of the invention that the dispersion device can be arotating bucket and the agitation device 17 is stationary.

The viscosity of the shear-thinned mixture at the particle-formingsection is important to the rate and particle characteristics of theprilling or spray-drying operation. This viscosity can be calculated andthus controlled by backward solution of the classic Fanning or Darcyequations coupled with Bernoulli's equation, considering the static headas measured by level gauge 14 as determined by the flow rate of materialentering through conduit 13, the pressure of the inert gas as measuredby pressure gauge 16, and the shear-thinning agitation effected byagitation device 17, and the configuration and resulting hydrauliccharacteristics of the particle-forming section; the Example describedhereinafter shows the results of such calculation. For a given prillingor spray-drying operation, a viscosity increase must be compensated byan increase in gas pressure or liquid level to maintain a given rate ofparticle formation; the viscosity then may be adjusted by varying theshear-thinning agitation. Depending on the specific characteristics ofthe desired particles and the type of viscometer used in measurement,the viscosity generally should be in the range of from about 5 to about6000 cp (centipoises), and more usually within the range of from about10 to about 600 cp.

FIG. 2 is a schematic illustration of a cross-section of a prillingembodiment of the invention. A prill head 20 contains a fluid from whichparticles are to be formed as well as a pressure gauge, a shear-thinningapparatus, and a prill-forming section. The fluid, having a surfacerepresented by 21, enters the apparatus through conduit 22. The level ofthe fluid surface in the device is controlled between upper and lowerlimits established by level gauge 23 between the entry conduit and theshear-thinning apparatus. An inert gas, for example nitrogen, isprovided through conduit 24 and determines pressure in the apparatusthrough pressure gauge 25. Motive device and shaft 26 are, e.g., avariable-frequency-drive motor which rotate the shear-thinning apparatuscomprising high-shear mixing blades 27 and optional floating wiper 28.The floating wiper provides final high-shear agitation for small prills;very small prills require that the blade 27 has zero clearance aboveprill-head sizing device 29 in order to wipe the plate. The prill-headsizing device 29, or prill plate, is held in place by prill-head flange30 and head-holding flange 31, yielding prills 32.

In all embodiments of the agitated prill head, the prill holes should bespaced sufficiently apart from each other to preclude the descendingstreams and/or prills from touching one another and coalescing. Prillhole diameters may be any size commonly practiced in the art. Prill holediameters of about 2.0 to about 4.0 mm are particularly useful in theinventive process and apparatus. Smaller prill hole diameters, of lessthan 2.0 mm, may be used in an agitated prill head and may utilizesurface-wiping blades to wipe the prill plate.

FIG. 3 is a schematic illustration of a cross-section of a spray-dryingembodiment of the invention. A spray-nozzle body 50 contains a fluidfrom which particles are to be formed as well as a pressure gauge, ashear-thinning apparatus, and a spray-granulation section. The fluid,having a surface represented by 51, enters the apparatus through conduit52. The level of the fluid surface in the device is controlled betweenupper and lower limits established by level gauge 53 between the entryconduit and the shear-thinning apparatus. An inert gas, for examplenitrogen, is provided through conduit 54 and determines pressure in theapparatus through pressure gauge 55. Motive device and shaft 56 are,e.g., a variable-frequency-drive motor which rotate the shear-thinningapparatus comprising high-shear mixing blades 57 and final high-shearagitator 58. A restrictor 59 is held in place by body flange 60 andhead-holding flange 61, sending the sheared fluid to a spray head, or apressurized nozzle assembly 62.

In spray drying, the fluid from the nozzle is dispersed into a stream ofhot gas in a mist of fine droplets. Moisture is vaporized from thedroplets to leave residual particles of dry solid which are separatedfrom the gas stream. The flow of fluid and gas may be cocurrent,countercurrent or a combination of both. Further details of spray dryingmay be found in Perry, J. H. et al., CHEMICAL ENGINEERS' HANDBOOK,7^(th) ed., McGraw-Hill Book Co., Inc., New York (1997), or in Masters,K., SPRAY DRYING, AN INTRODUCTION TO PRINCIPLES, OPERATIONAL PRACTICEAND APPLICATIONS, 2^(nd) ed., G. Godwin Ltd., London, J. Wiley & Sons,New York (1976), both of which are incorporated herein by reference.

Generally, any mixture of components that yields a shear-thinnablemixture may be used in the inventive method. The invention isparticularly useful in forming prills or granules of ammonium sulfatenitrate, the double salt of ammonium sulfate and ammonium nitrate As themolten first component, ammonium nitrate is most preferred. Othersuitable molten first components include urea and ammonium phosphates.Suitable second components may include any material which results in aviscous and shear-thinnable mixture when added to the molten firstcomponent. The second component may or may not be fully meltable and/orsoluble in the molten first component. As the second component, ammoniumsulfate is most preferred. Useful ammonium sulfate is commerciallyavailable from Honeywell International Inc., Hopewell, Va., USA. Othersuitable second components include potassium chloride. Other materialscan be added to the shear-thinnable mixture if desired as long as theydo not adversely affect prilling. For example, possible third componentsinclude micronutrients such as iron sulfate, magnesium sulfate, boronsalts, and anti-caking agents.

Temperature restrictions on the reaction are dictated by the componentsused. One needs to uses a temperature range in which the first componentmelts without problems of decomposition or deflagration. When usingammonium nitrate, the minimum temperature is about 180° C. (the meltingpoint of ASN) and a safe maximum reasonable handling temperature isabout 200° C. Preheating the second component before addition to themolten first component is useful in mixtures where the heat of reactionis important, and is generally desirable due to heat transfer.

Generally, addition of water to a mixture to be prilled is minimized toallow solidification of the prills without the need for removal ofexcess solvent. The water addition aids melting and suppresses fuming ofammonium nitrate. Addition of water to the molten mixture is preferredto be no more than about 2.0 weight percent (wt-%) and is preferablyless than or equal to about 1.0 wt-% and more preferably less than orequal to about 0.5 wt-%. Addition of water from about 2 to about 6 wt-%is possible; however, it detrimentally impacts prill strength, andrequires the prills to be dried.

One of the benefits of the inventive process is that it allowssufficient reaction time to develop advantageous properties in theproduct. For instance, pure ammonium nitrate prills are problematic forseveral reasons: 1) storage problems result from its hygroscopic nature;2) a phase transition at 32° C. can cause the prills to break apart asthe temperature fluctuates (“sugaring”); and 3) it is an oxidizer. Incontrast, using ammonium nitrate and ammonium sulfate in the inventiveprocess results in a double salt ASN product that has vanishingly littleunreacted ammonium nitrate. This product possesses many favorableproperties including: 1) reduced hygroscopic problems; 2) no “sugaring”;and 3) ASN is not an oxidizer.

EXAMPLE

A spreadsheet was developed applying the classic Fanning equationcoupled with Bernoulli's equation as applied to the following parametersto calculate the viscosity of a shear-thinned mixture at a prill head:

Prill head: Number of machined holes 100 Estimated blocked holes 11Number of operating holes 89 Diameter of holes 3.6 mm Cross-sectionalarea of each hole 10.18 mm² Thickness of prill head at hole 1.9 mmEntrance of fluid to hole sharp-edged Entry coefficient 0.5 Fluidproperties: Specific gravity at operating temperature 1.6 Operatingconditions: Height of liquid in shearing device 0.67 meters Pressureabove liquid in shear device 11.0 kPa Pressure at prill head 2.198meters of water Pressure inside prilling tower −10 cm of water Flow rateof liquid to prilling head 0.2196 m²/minute Calculated conditions:Differential pressure across prill head 2.30 meters of water Flow rateof liquid to prilling head 0.2196 m³/minute Mass flow to prilling head350.6 kg/minute Mass flow to prilling head 21,038 kg/hour Average flowthrough each hole 3.9397 kg/minute Average flow through each hole 236.38kg/hour Average flow through each hole 0.002467 m³/minute Velocitythrough prilling hole 248.4 meters/minute Velocity through prilling hole4.14 meters/second Pressure drop: Velocity pressure inside prilling hole0.872 meters of water Pressure drop from prilling hole length 0.991meters of water Total pressure drop with orifice entrance 2.30 meters ofwater Total pressure drop available 2.30 meters of water Viscosity ofprilling fluid 48.7 cp

1. A method of calculating and controlling the viscosity of ashear-thinnable fluid stream at a particle-forming section of adispersion device by: a) providing a measured static head ofshear-thinnable fluid within the dispersion device; b) providing apressurized blanket of inert gas over the fluid at a measured gaspressure; c) mechanically agitating the fluid in the dispersion deviceat a measured agitation speed; d) calculating the viscosity of theagitated fluid; and e) controlling the viscosity at the particle-formingsection by adjusting the agitation speed.
 2. The method of claim 1wherein the dispersion device is a prill head and the particles areprills.
 3. The method of claim 1 wherein the shear-thinnable liquid isformed by providing a molten first component, mixing at least one secondcomponent with the first component, and reacting the components at atemperature and for a time sufficient to form a mixture having aviscosity which decreases with increasing shear rate.
 4. The method ofclaim 3 wherein the shear-thinnable mixture is a melt slurry.
 5. Themethod of claim 1 wherein the shear-thinnable liquid comprises no morethan about 2 weight percent water.
 6. The method of claim 1 wherein thedispersion device is a spray nozzle.
 7. The method of claim 6 whereinthe liquid is a slurry.
 8. A method of calculating and controlling theviscosity of a shear-thinnable fluid stream at a prill head by: a)providing a measured static head of shear-thinnable liquid within theprill head; b) providing a pressurized blanket of inert gas over theliquid at a measured gas pressure; c) mechanically agitating the liquidin the prill head at a measured agitation speed; d) calculating theviscosity of the agitated liquid; and e) controlling the viscosity ofthe liquid at the prill head by adjusting the agitation speed.
 9. Themethod of claim 8 wherein the shear-thinnable liquid is formed byproviding a molten first component, mixing at least one second componentwith the first component, and reacting the components at a temperatureand for a time sufficient to form a mixture having a viscosity whichdecreases with increasing shear rate.
 10. The method of claim 9 whereinthe shear-thinnable mixture is agitated in the prill head underconditions such that the entire liquid volume in the prill head is sweptby the agitator to shear-thin the shear-thinnable mixture.
 11. Themethod of claim 9 wherein the shear-thinnable mixture is a melt slurry.12. The method of claim 9 wherein the first component is ammoniumnitrate and the second component is ammonium sulfate.
 13. The method ofclaim 9 wherein the shear-thinnable mixture further comprisesmicronutrients.
 14. The method of claim 9 wherein the shear-thinnablemixture comprises no more than about 2 weight percent water.
 15. Themethod of claim 8 wherein the prill head is one selected from the groupcomprising a rotating bucket with a stationary blade, a stationarybucket with rotating scrappers and blades, and an agitated pressurizednozzle assembly.
 16. The method of claim 8 wherein the prill head iswiped with surface-wiping blades.
 17. A method of calculating andcontrolling the viscosity of a shear-thinnable fluid stream in a spraynozzle by: a) providing a measured static head of shear-thinnable fluidwithin the nozzle; b) providing a pressurized blanket of inert gas overthe fluid at a measured gas pressure; c) mechanically agitating thefluid in the nozzle at a measured agitation speed; d) calculating theviscosity of the agitated fluid; and e) controlling the viscosity of thefluid by adjusting the agitation speed.
 18. The method of claim 17wherein the liquid is a slurry.
 19. The method of claim 17 wherein thenozzle is one selected from the group comprising a rotating bucket witha stationary blade, a stationary bucket with rotating scrappers andblades, and an agitated pressurized nozzle assembly.